The present invention relates to wet scrubbers, and in particular to a new and useful technique for monitoring and controlling forced oxidation in a wet scrubber.
In situ, forced oxidation is a process used in limestone wet scrubbing Flue Gas Desulfurization (FGD) to produce a waste that is primarily gypsum. This process involves the introduction of compressed air into a wet scrubber slurry reaction tank to provide sufficient oxygen for the formation of gypsum. The chemical name for gypsum is calcium sulfate dihydrate (CaSO.sub.4 *2H.sub.2 O). If air is not added to the wet scrubber, then significant amounts of calcium sulfite hemihydrate (CaSO.sub.3 *1/2H.sub.2 O) will form and result in a waste that is more difficult to dispose of. Limestone wet scrubbers chemically react a mixture of finely ground limestone and water with sulfur dioxide to produce a solid waste product.
In situ, forced oxidation is widely used in limestone wet scrubbers in the electric utility power generation industry. To produce a commercial grade gypsum product, sufficient compressed air must be added to the wet scrubber to maintain the oxidation above 99%. Oxidation is measured by chemically analyzing the wet scrubber slurry for sulfite (SO.sub.3 =) and sulfate (SO.sub.4 =). percent oxidation is expressed as the molar ratio of sulfate to total sulfur species: ##EQU1##
The current method of controlling oxidation is to obtain a sample and conduct a chemical analysis. Typically, the results of the chemical analyses are completed on the same day that the sample was obtained. There is no real time measurement to indicate loss of gypsum production used in prior art FGD systems.
The current method of controlling oxidation is to operate the compressed air supply system at full capacity during all periods of wet scrubber operation. In other words, the oxidation air flow is not controlled, but maintained at the design flow rate. This control method, while ensuring that sufficient compressed air is added even under the most extreme design operating conditions, wastes energy in the form of compressor power under all other operating conditions.
During process upsets, which may not be discovered until several hours after they begin, the highest air flow possible is used to recover from the upset. Until high oxidation occurs, the solid product quality would be unacceptable and must be kept separate from the quality gypsum. Otherwise, the unacceptable quality waste may adversely affect the handling and value of the quality gypsum. The unacceptable quality waste can be disposed of, but this involves additional cost and handling.